U.S. Department of Transportation Federal Aviation Administration

Circular

Subject: TIEIDWN SENSE

Date: 7/12/83 AC No: 20035C Initiated by: AWS - 340 Change:

1 l PURPOSE. This advisory circular provides updated information of general use for aircraft tiedown techniques and prooedures.

2 . CANCELLATION. AC 20-35E3, TIE-DUiilN SENSE, dated April 19, 1971, is canceled.

3 l J3ACKGROUND. Each year numxous aircraft are medlessly damaged by windstorms because of inattention to weather forecasts, negligence, or improper tiedown procedures. WindstorMs may be broadly classified as cyclonic storms lm pressure systems, regional or localized terrain induced winds, thunderst or

4 . STORMS,

or OlTlS

a. Cyclones. Cyclones are the lows of the weather maps. In the United States the nama does not suggest any degree of intensity in the purest mzteoro- logical sense and is applied to &rate as well as intense storms. Winter storms are atmospheric disturbances that may becm intense low pressure systems churning over tens of thousands of square miles. 'In our northland the severity of these storms makes them seasonal threats. These intense low pressure systems combine winds s-times as strong as 90 mph with snow and cold temperatures. The National mather Service (NWS) issues timely watches and warnings against the hazards of winter weather so that persons in the alerted area may take precautionarv treasures.

AC’20-35C 7/12/83

b Regional or Localized Terrain Induced Winds. Geography is a factor in producing local and smtirtes strong winds due to peculiar or unique terrain effects. The Santa Ana winds of Southern California, or the winds in Boulder, Colorado, are examples of infrequent localized winds that are generally forecast far enough in advane to minimize the potentially damaging effects upon parked aircraft.

C. Thunderstorms. Individual thunderstorms may measure from less than 5 miles to m3re than 30 miles in diaraeter, and may occur both in isolation and as a part of larger weather systems. In sol places and seasons thunderstorms recur almost daily at almost the same time. In other places they occur rarely or irregularly. SCEE last a few minutes and on other occasions a succession of related events may last most of the day. The significant features of thunder- storms are high wind velocity, lightning, intense precipitation and hail. All these are variable features that appear in many combinations. Strong straight- line winds accompany thunderstorms rr~re often than tornadoes and TMY be as damaging to persons and property as small tornadoes. Strong and shifting winds along thunderstorm gust fronts have been associated with tragic accidents to commercial aircraft.

Average number of

The map above shows the yearly average number of days with thunderstorms based on observations in the U.S. A thunderstorm day is considered any day during which one or mr>re thunderstorms occur. It should be realized, however, that there are local variations which do not show on this map because of the sparsity of observations from SCE areas. July and August are the rrw>nths with the greatest number of thunderstorms over most sections of the U.S. while I&ember and January have the least number.

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7/12/83 AC 20-3X

d. Hurricanes. mar&l to the great cyclonic storms of the Temperate Zone, hurricanes are of -rate size and their worst winds do not approach tornado velocities. Their winds exceed 70 qh and may reach ZIO mph, and their lifespan is treasured in days or weeksr not minutes or hours. No other atmos- pheric disturban- ccmbines durationr size, and violence more &structively. Hurricanes are a threat to the Gulf and East Coast during the hurricane season from June through November. Decaying tropical storms may on rare occasions mve inland and dump flash-flood producing rainfall over California and the desert Southwest, Timly detection of and warning against hurricanes has been the task of the NWS for nearly a century.

(1) Hurricane Camile slmd into the Northern Gulf Coast near Christian, Mississippi, on the night of August 17, 1969. Camile's 200 mph winds produced a devastating tidal storm surge of nearly 25 feet just east of where the center mde landfall. Camile ranked as one of the rrw>st destructive storms ever to strike the U.S. Total damage was about 1.4 billion dollars with 256 deaths credited to the hurricane alonq the Gulf Coast. me remnants of the dying storm produced killer flash floods in Virginia.

(2) Hurricane Frederic in September 1979 brought 160 mph winds and 12-foot tidal storm surges to a densely populated area in and rzear Mobile, Alabama. Destruction was widespread and damage was estimated at rtaore than 2 billion dollars. Precise forecasts and warnings, together with prqt local action, permitted about a quarter of a million people to evacuate to safe areas. The loss of life was extremly low -- five people died. President Carter credited the low death toll of Hurricane Frederic to accurate and tiely warnings coupled with orderly and extensive evacuation.

e. Tornadoes. Tornadoes are fears- storms that usually move frm the southwest. The more severetornadoes mve at forward speeds of about 60 mph accunpanied by winds of 200 mph or more. Since the tornado core is rarely larger than .6 mile in dieter, destructive winds generally last at m place for less than a minute but practically all structures and Itlklch of the natural environment can be destroyed by severe tornadoes in just a few seconds. More

because of in path. ies to the

intense than the worst hurricanes, tornadoes mnd much attention their sudden and violent onslaught and occasional sharp alteration Storm forecasting and warning operations (including weather advisor aviation public) help reduce the harmful effects of storms.

‘AVERAGE NUMBER OF TORNADOES AND ADO DEATHS BY STATE 1953 - 1980

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AC 20-35C 7112183

TORNADOES, DEATHS, AND TORNADO DAYS 1953.1980

564 550

95’ -

61 -

a34

A4

f&i

976 -

67 -

1200

3100

1000

900

a00

700

600

500

%7

&

1959 - 159 -

968 1969 1970 1971 1972 1973 1974 1971 2 TORNADO INCIDENCE BY MONTH 19534980 16C

140

80

60

40

I I \ I TORNADOES

\

I I i/

81 \

\ \

TORNADO DAYS

65 \

\ \

\ \

38 \

JAN FE6 MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

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7112183 AC 20035C

Average Annual Tornado Incidence Per 10,000 Square Miles

1953 - 1980

MONTHS OF PEAK TORNADO ACTIVITY

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.

AC 20035C 7112183

STORM BELTS

- -2. / -i: , c - -‘y-. I --T__-- -- i i i

\ \ .?-- : i -------+- - - --_ \ -1

84 Prima xy hurricane danger zone.

$f)/$ Fringe area.

Tornado belt.

HURRICANE DAMAGE

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7112183 AC.20-35C

5 l PREVENTINGWI~IYXM IY!MAGEe The best protection against windstorm damage is, of courser to fly the aircraft out of the impending storm area provided you have sufficient warning time. The next best protective masure is to secure the aircraft in a stormproof hangar or other suitable shelter, The remaining alternative is to assure that the aircraft is tied down securely. men securing your aircraft, it is considered good practice to fasten all doors and windows properly, thereby minimizing damage inside the aircraft. Engine openings (intake and exhaust) for both reciprocating and gas turbines should be covered to prevent entry of foreign matter. Pitot-static tubes should also be covered to prevent damage or entry of foreign matter, Make sure your neighbor's aircraft is also tied down,

6 ADVANCED PLANNING. It is the mission of the NWS to help mitigate the threat to life and property frcan natural hazards through the issuance of tornado and severe thunderstorm watches and warnings. NWS meteorologists at the National Severe Storms Forecast Center (NSSFC) monitor atmospheric conditions utilizing information frm many sources and locations. When hazardous conditions are anticipated or detected, watches or warnings are issued.

a. Watches are issued by the NSSFC to indicate when and where severe thunderstorms and/or tornadoes are rrw>st likely to occur. Watches are usually issued for areas about 140 miles wide, ZDO miles long and generrally 2 to 4 hours in advance of severe weather, Listen to the National Gceanic and Atmospheric Administration (NQAA) weather radio (162.400-162.55OMHZ) continuous broadcasts for the latest weather information directly fram NWS offices, and use mrcial radio or television for further information.

b Warnings are issued by local NWS offices when severe thunderstorms or tornadoes are indicated by weather radar I weather observers or trained spotters. A warning describes an tinent risk frcxn a tornado or severe thunderstorm in a relatively small area such as one or several counties, The key to damage avoidance or reduction is to be routinely mather conscious.

Ce Be prepared for the worst conceivable windstorm conditions: pouring rain, gusty winds ranging frcan 30 wh and up, for example intermittent sheets of water blowing across the runways, ramps, and parking areas, and lack of hangar facilities. With such conditions in mind, aircraft owners and operators should plan in advance by learning their aircraft manufacturer's instructions for tiedown; location and/or installation of tiedown rings for attachrolent of tiedown ropes; any special instructions for securing nosewheel type aircraft VSe tailwheel type aircraft; and manufacturer's charts and graphs denoting aircraft weights and relative wind velocities that would make varied tiedown procedures necessary for pending weather emrgenciese

7 l TIEm FACILITIES. Any aircraft parking area should be equipped for three-point tiedowns. Aircraft should be tied down at the end of each flight to PIZC~I.& damage fran sudden storms. 'Ihe direction in which the aircraft are to be parked and tied down will be determined by prevailing or forecast wind direction.

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AC 20035C 7112183

a. Aircraft should be headed into'the wind, or as nearly as possible, depending upon the locations of the fixed parking area -ring points.

i I

MAJOR AXIS

b Spacing of tiedowns should allow for ample wingtip cleararm. Spacing should be equal to the major axis (wingspan or fuselage length) of the largest aircraft usually operated plus 10 feet.

After the aircraft is properly lock Tn the forewandcaft position. TTTi

located, the nosewheel or the tailwheel

(1) Tiedown anchors for single-engine aircraft should provide a minimum holding power (strength) of approximdtely 3,000 pounds each. The type of anchors in use varies depending upon the type of parking area- whether for a concrete paved surface, a bituminous paved surface, or an unpaved turf area. Location of tiedowns are usually indicated by soar suitable mans, either white or yellow paint I or a painted tire which has been fastened into the ground, or surrounding the tiedown anchor with crushed stone. The tiedown anchor eye should not protrude more than 1 inch above ground.

TIEDOWN ANCHORS FOR CONCRETE PAVED AREAS

I ‘/e* MIN. \ tp” TO 5”+ / TROWELED DEPRESSION

CASTING - MALLEABLE IRON\ p 6” Dia. {CONCRETE PAVEMENT

SECTION A-A

r - ~~ -

I ‘/g” MIN. -y, 4” TO b S;uTROWELED DEPRE SSION

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.

7/12/83 AC 20-35C

TIEDOWN ANCHOR FOR

Bl%,NO”S PAVED AREAS

I ‘/oU WEARIN

3” BITUMINOUS BASE

TIEDOWN ANCHORS FOR TURFED AREAS

REINF. STEEL ROD

f-0” SQUARE TE BLOCK

I’&” MIN. EYE

,GROUND LINE

//; DIA. ROD

DISCARDED FARM g DISC BLADE

-BOLTED THROUGH DISC

ROD AND BLOCK ANCHOR MUSHROOM TYPE ANCHOR

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AC 20-3X 7/12/83

(2) Stake-driven tiedowns such as depicted above will almost invariably pull out when the ground becanes soaked from torrential rains which accompany hurricanes and scme thunderstorms.

(3) Tiedown ropes capable of resisting a pull of approximately 3,000 pounds should be used. Manila ropes should be inspected periodically for mildew and rot. Nylon or dacron tiedown ropes are preferred over manila ropes. The objection to manila rope is that it shrinks when wet, is subject to mildew and rot, and has considerably less tensile strength than either nylon or dacron.

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